On-board nitrogen generation systems can be used to create an inert environment in various locations on an aircraft, such as in the cargo area or fuel tanks. Nitrogen generation systems can produce nitrogen-enriched air using molecular sieve technology or hollow fiber membrane technology. For example, an air separation module containing a plurality of hollow fiber membranes separates compressed air into a nitrogen-enriched portion and an oxygen-enriched portion. The compressed air source can be compressed RAM air or can be bleed air taken from the compressor section of the aircraft engine. A control unit can control the purity or oxygen concentration of the nitrogen-enriched air. For example, a flow control valve can be located downstream of the air separation module to control the flow of air through the air separation module. A faster flow rate through the air separation module results in the nitrogen-enriched air having a lower purity (higher oxygen concentration).
The nitrogen-enriched air produced by the nitrogen generation system can be directed to the fuel tanks of the aircraft. Nitrogen-enriched air is supplied to the fuel tanks in a sufficient quantity to maintain the oxygen concentration in the fuel tank below a specified amount in order to limit the flammability of the fuel tanks.
Oxygen sensors run at high reference temperatures and thus cannot be used to directly measure the oxygen concentration of the fuel tank because of the risk of ignition. Instead, typically an oxygen sensor is located upstream of the fuel tank to measure the oxygen concentration of the nitrogen-enriched air stream flowing into the fuel tank. The sensed values are used to approximate the actual oxygen concentration of the fuel tank based on established models, and the control unit can adjust the oxygen concentration of the nitrogen-enriched air flow based on this feedback.